Our objectives are the eluciation of the collagen-platelet interaction which initiates primary hemostasis. To this end, our plan is to investigate, in vitro, model systems in which the soluble collagen and the platelets can be examined in detail. The initial purpose was to identify, by physical-chemical means, the requisite functional characteristics of collagen: Structure, conformation, functional groups, carbohydrate residues, length, macromolecular state. A second goal was to attempt parallel identification of the requisite components of the platelet surface. A third objective was to apply all of these studies to the adhesion reaction of platelets to collagen as well as to the aggregation reaction which follows in many cases. Our fourth and ultimate aim was to develop means of designing artificial conditions, or of modifying either or both of the components such that, in vitro (and hoepfully later in vivo) the formation of platelet aggregation could be facilitated, as in some hemorrhagic disorders, or conversely, prevented, as in some diseases of the circulatory systems where premature or excessive formation of platelet aggregates (often circulating in the blood stream) constitute a clinical problem.